This laboratory has been engaged in the isolation and purification of histocompatibility antigens of the rabbit. There are at least eight serologically defined haplotypes in the domestic rabbit. Current work has concentrated on the RL-Ac haplotype. The major sources of antigen have been peripheral blood lymphocytes from Bar Harbor B-strain rabbits and a tumor cell line derived from this inbred rabbit strain. Antigen isolation has been carried out by detergent solubilization of cell membranes that were intrinsically labelled with tritiated amino acids. Glycoproteins have been isolated and analyzed by SDS-polyacrylamide gel electrophoresis indicating that there are two major classes of antigens. The first has a molecular weight of 43,000 and has associated with it a beta-2 microglobulin (B2m) sub-unit. The second antigen type has an apparent molecular weight of 32,000. Structural studies of rabbit B2m are currently underway.